Fly ash is a waste discharged from the coal-fired power station. In China, the coal is used as one of the main energy sources and hundreds of millions tons of fly ash are discharged from power stations each year. The discharge of the fly ash not only occupies a large area of land, but also pollutes the environment seriously. How to handle and utilize the fly ash becomes a very important problem. The fly ash contains a number of components that can be utilized, for example, circulating fluidized-bed fly ash usually contains about 30 to 50 percent by weight of alumina. Nowadays, as the bauxite resources become gradually rare, it is an efficient way to extract valuable materials, such as alumina from the fly ash and provide a highly comprehensive utilization for the fly ash, which is of great social and economic benefits.
In light of different conditions of calcinations, the fly ash is classified into pulverized coal-fired boiler fly ash and circulating fluidized-bed fly ash. The pulverized coal-fired boiler fly ash is produced when coal is burned at very high temperatures (1400-1600° C.), wherein the component of alumina is in glassy state or present as mullite crystals or corundum crystals. While the combustion temperature of circulating fluidized-bed fly ash is much lower than that of traditional pulverized coal-fired boiler fly ash, only about 850° C. Different combustion temperatures make a substantial difference in phase composition between the pulverized coal-fired boiler fly ash and circulating fluidized-bed fly ash, that is, amorphous kaolinite enters into the main phase composition of the circulating fluidized-bed fly ash, in which silicon dioxide, alumina and ferric oxide possess excellent activity.
By now, methods used for extracting alumina from fly ash are basically divided into two categories, acid leaching method and alkali leaching method. Further, the alkali leaching method may be divided into limestone (soda-lime) sintering method and sodium carbonate sintering method.
The limestone (soda-lime) sintering process is such a method that the fly ash is mixed with limestone (soda-lime) and the mixture is calcined to be activated at a very high temperature (1320-1400° C.), thereby alumina and silicon dioxide contained in the fly ash react respectively with limestone (soda-lime) to form calcium aluminate and dicalcium silicate. The calcined ash was leached with sodium carbonate solution and filtered to remove unwanted substances, thus the calcium aluminate enters the solution in the form of sodium metaaluminate, from which, after desilicification and carbon precipitation (or seed precipitation) treatments, aluminum hydroxide is thereby obtained. Subsequently, the obtained aluminum hydroxide is calcined to obtain alumina product. Moreover, after the filtering treatment, the dicalcium silicate turns into silicium-calcium slag which may be employed as raw material for cement. In CN 101070173A, CN 101306826A, CN 101049935A, CN 101302021A, CN 101125656A, CN 101041449A, CN 1030217A, CN 1644506A, CN 101028936A, CN 1548374A, CN 101085679A, CN 1539735A, for example, the limestone (soda-lime) sintering method or improved limestone sintering method is employed respectively. The sintering raw material used for the limestone sintering method is cheap limestone, which relatively reduce the production cost of alumina. However, there are many disadvantages exist in this process. Firstly, large quantities of silicium-calcium slag is produced during the leaching process. About 8-10 tons of silicium-calcium slag is produced for every ton of alumina obtained. Such silicium-calcium slag is prone to be another discharge of waste which even requires more occupancies if the slag can not be consumed thoroughly by the building materials market. Further, the silicone dioxide is not utilized with a high value. Secondly, the energy consumption in the limestone sintering process is very high, since the fly ash is calcined in a very high temperature, and the requirements on its procedures and equipments are also high. Thirdly, the recovery efficiency of alkali is very low as large quantities of slag are produced during the alkali leaching process, which increases the production cost.
The sodium carbonate sintering method is such a method that the fly ash and sodium carbonate are calcined at a high temperature (750-1450° C.), during which, alumina and silicon dioxide contained in the fly ash are activated simultaneously, thus the calcined fly ash needs to be further acidized through carbonation reaction or reacting the calcined fly ash with sulfuric acid/hydrochloric acid to separate silicone and aluminum. Such method, as alkali leaching comes before acid leaching, is also referred to as acid-alkali combination method. For example, CN 101041450A, CN 101200298A, CN 101172634A, CN 101254933A have described the sodium carbonate sintering method. As compared with the limestone sintering method, less slag is produced in the sodium carbonate sintering method and silicone dioxide in the fly ash can be utilized with a high value. However, the energy consumption of the process is high and its procedures are complicated since the fly ash needs to be calcined in a very high temperature and the calcined ash needs to further act with acid in order to separate silicone and aluminum.
As described above, in both limestone sintering method and sodium carbonate sintering method, the fly ash needs to be activated by reacting with limestone/sodium carbonate at a very high temperature. Consequently, such methods are suitable for pulverized coal-fired boiler fly ash which has weak activity. Whereas, the circulating fluidized-bed fly ash can react with acid without activation due to its higher activity.
The acid leaching method is such a method that the fly ash react with acid solution directly to obtain aluminum-salt solution and the aluminum-salt is calcined, so as to be decomposed to prepare alumina For example, CN 1923695A, CN 1920067A, CN 101045543A, CN 101397146A, CN 1792802A, CN 1927716A have described that alumina is extracted from fly ash by use of the acid leaching method. The fly ash usually reacts with acid at a temperature lower than 300° C. As compared with the high calcination temperatures used for the limestone sintering method and sodium carbonate sintering method, the energy consumption of the acid leaching method decreases dramatically. Moreover, all silicone dioxides will be kept in the solid residues since it does not react with acid. Furthermore, some impurities, such as calcium and sodium, will not be introduced in the alumina product. As such, it is possible to obtain high purity alumina theoretically. However, one of the defects of the acid leaching method resides in that soluble impurities, such as iron, will enter the solution, resulting in that the obtained alumina contains quite a number of iron and the like which are difficult to be removed. One solution to this is to dissolve the alumina obtained via the acid leaching method by alkali, such that aluminum is turned into sodium metaaluminate and enters solution, while the impurities, such as iron, will precipitate in form of hydroxide, such that the iron is removed from the alumina product. Since acid dissolving and alkali dissolving are both necessary in such method, its processes are complicated and its production cost is increased.